The desired compounds (referred to as a group hereinafter by the expression "Fl20 series") are: F121 (CHCl.sub.2 --CCl.sub.2 F), F122 (CHCl.sub.2 --CClF.sub.2), F123 (CHCl.sub.2 --CF.sub.3), F124 (CHClF--CF.sub.3), F125 (CHF.sub.2 --CF.sub.3) or their isomers which can be employed either as substituents for perfluorocarbons (C.F.C.) in the fields of foams (blowing agents and insulants), aerosols (propellent agents) or in refrigeration, or else as intermediates for the synthesis of these substitutes. Efficient processes for their industrial production, and more particularly for that of F124 and of F125 are sought after at present.
Patent FR 1,315,351 describes a gas phase process for fluorination of haloolefins with a catalyst prepared by partial fluorination of an alumina impregnated with one or a number of halides of polyvalent metals such as chromium, nickel, cobalt, manganese etc. The authors stress the partial fluorination of the catalyst because it turns out, in their case, that an advanced (&gt;80%) fluorination of the said catalyst results in a rapid loss of activity. The tests of Example 4 describing the fluorination of perchloroethylene are carried out with a catalyst based on chromium and cobalt derivatives deposited on Al.sub.2 O.sub.3. At low temperature (T&lt;290.degree. C.), despite an HF/C.sub.2 Cl.sub.4 molar ratio of 5, the inventors obtained a mixture of F121, F122, F123 and of the olefins F1111 (CFCl.dbd.CCl.sub.2) and F1112a (CF.sub.2 .dbd.CCl.sub.2). In the case of this mixture it is noted that F123 is clearly predominant but apparently the authors do not observe the formation of F124, F124a (CF.sub.2 Cl--CHF.sub.2) and F125. Furthermore, it is concluded that the selectivities for olefins are high already at low temperature and increase with the latter.
Patent Application WO 8911467 claims the preparation of F123 and/or of F124 by gas phase fluorination of perchloroethylene using HF in the presence of a catalyst comprising a metal--in an oxidation state higher than zero--chosen from the group consisting of Cr, Co, Mn, Rh and Ni on an alumina support, highly fluorinated. In contrast to the preceding patent it appears necessary to fluorinate the catalyst strongly (AlF.sub.3 &gt;90%) before going on to fluorinate the organic products. The object of the process claimed is to produce F123 and F124 while minimizing the formation of F125. The authors' preferred metal is cobalt; however, according to the examples, it is found that, whatever the metal tested: Cr, Co, Mn or Ni, and despite high temperatures and HF/C.sub.2 Cl.sub.4 molar ratios, the selectivities for F125 remain very low (&lt;10 %). Furthermore, numerous byproducts (outside the F120 series) are formed; in particular, with chromium or nickel the selectivities for F123+F124+F125 do not exceed 85-87% when operating at 350.degree. C.
Patent Application JP 2-178237 describes the gas phase fluorination of perchloroethylene on bulk catalysts consisting of chromium oxides and of at least one of the following metals: Al, Mg, Ca, Ba, Sr, Fe, Ni, Co and Mn. Despite quite high temperatures (350-380.degree. C.), the bulk catalysts exemplified (cometal Al, Mg, Ca, Ba, Sr or Fe) result in high selectivities for F122, whereas those for F125 remain lower than 15%. Now, as indicated in Patent Application FR 2,661,906, F122 is not an intermediate of great interest for the synthesis of F123-F124-F125 because it gives rise to numerous byproducts, especially olefins.
Patent Application EP 366 797 claims a process for fluorination of saturated or unsaturated compounds in the presence of catalyst based on porous and very pure alumina (containing less than 100 ppm of sodium), used as support for metal fluorides (nickel, cobalt, iron, chromium, manganese, copper or silver). This technique, which requires a catalyst of high purity, is illustrated in the case of the F120 series (pentahaloethanes) by two examples (27 and 30) with a chromium-based catalyst supported on alumina. In one case F123 is obtained predominantly, but in low yields (20%), by gas phase fluorination of C.sub.2 Cl.sub.4 ; in the other case, depending on the reaction temperature, it is F124 or F125 that predominates, but this time the underlying reaction is fluorination (addition of HF and then Cl/F exchange) of F1113 (CClF.dbd.CF.sub.2).
Patent Application FR 2,661,906 relates to the synthesis of F124 and of F125 by gas phase fluorination of F123 with a chromium-based catalyst deposited on active carbon. On catalysts of this type F123 gives rise to few byproducts that cannot be turned to profit, such as the compounds F115 (CF.sub.3 --CF.sub.2 Cl), F114a (CF.sub.3 --CFCl.sub.2), F114 (CF.sub.2 Cl--CF.sub.2 Cl) and F133a (CF.sub.3 --CH.sub.2 Cl) or that can be detrimental to the catalyst life, such as the olefin F1111. On the other hand, it is indicated in this same patent that the underfluorinated compounds of F123 (F122, F121, C.sub.2 Cl.sub.4 etc.) are not good precursors for F124 and F125.
This finding is confirmed by U.S. Pat. No. 3,258,500, which claims the use of chromium, bulk or supported on alumina, for gas phase fluorination reactions. Thus, Example 17 (column 14) describes the fluorination of perchloroethylene at 400.degree. C. with an HF/C.sub.2 Cl.sub.4 molar ratio of 6.2/1; in this case the selectivity for F123+F124+F125 is low (47.7%). A decrease in the reaction temperature (300.degree. C.) changes this F123+F124+F125 selectivity only to 79.7% and in this case F125 is no longer the predominant compound.
Patent Application FR 2,669,022 describes the use of a catalyst based on nickel and chromium which are supported on AlF.sub.3 or fluorinated alumina for the specific fluorination of F133a (CF.sub.3 --CH.sub.2 Cl) to F134a (CF.sub.3 --CH.sub.2 F), this catalyst enabling very good selectivities for F134a to be obtained.